1. Field of Invention
Exemplary aspects of the present invention generally relate to a fixing unit and an image forming apparatus using the same, and more particularly, it relates to an image forming apparatus including a fixing apparatus capable of effectively maintaining fixability for an extended period of use.
2. Discussion of the Background
In related art toner image forming mechanisms of image forming apparatuses using an electrophotographic method, for example, an electrostatic latent image is formed on a front surface of a photoreceptor serving as an image carrier, and the electrostatic latent image on the photoreceptor is developed and visualized using a developer such as toner. Then, the developed image is transferred onto a recording medium by a transfer apparatus. The recording medium carrying an unfixed image is fixed by a fixing apparatus using pressure, heat, and so forth, so that the toner image is fixed on the recording medium. The recording medium on which the toner image is fixed is ejected outside the image forming apparatus. The fixing apparatus is provided with two rotary fixing members composed of rollers facing one another, or belts, or a combination of a roller and a belt. The two rotary fixing members nip the recording medium therebetween and apply heat and pressure so as to fix the toner image on the recording medium.
At a place where the two rotary fixing members nip the recording medium, two rollers are disposed facing each other through a belt, when a belt is provided. The two rollers push each other in such a manner that one of the elastic rollers having a relatively low stiffness among the two rollers is recessed compared with the other roller having a relatively high stiffness. Thereby, the fixing nip is formed. In addition, when the belt is provided, the two rollers push each other through the belt therebetween so as to form the fixing nip.
In recent years, along with a tendency to downsize an image forming apparatus, it is necessary to downsize the two rollers which form the fixing nip in order to achieve downsizing of the fixing apparatus. If a diameter of the rollers is reduced, a deformation amount at the fixing nip is reduced even if the stiffness of the elastic rollers is the same. If a width of the fixing nip is reduced, this might cause a reduction of fixability. As a fixing apparatus that secures the width of the fixing nip in spite of downsizing of the two rollers, there is a fixing apparatus using elastic rollers with low stiffness having a sponge-like elastic layer. According to Japanese Patent Laid-Open Application Publication No. 2005-49455 and Japanese Patent Application Publication No. 3506880, an elastic roller using a roller with low stiffness having a sponge-like elastic layer is proposed. In Japanese Patent Laid-Open Application Publication No. 2005-49455, an elastic roller having the stiffness of greater than or equal to 5 Hs and less than or equal to 40 Hs on the Asker C scale is proposed. In Japanese Patent Application Publication No. 3506880, an elastic roller having the stiffness of greater than or equal to 10 Hs and less than or equal to 50 Hs on the Asker C scale is proposed. When using the elastic roller with low stiffness, the width of the fixing nip is secured, and the fixability is maintained even if the two rollers are downsized.
However, in the related arts, when the elastic roller with the low stiffness having the sponge-like elastic layer is used for a long period of time, the width of the fixing nip may be reduced. The possible reason for the reduction of the width of the fixing nip may be considered as follows.
In the elastic layer, many holes or cells separated by walls are formed so as to create the sponge-like layer. In the case where the cells are compressed and collapsed, the surface elastic force is obtained due to the resilience of the walls forming the cells, thereby being able to obtain a desired nip pressure. If the pressure is repeatedly applied to the sponge-like elastic layer, the walls forming the cells may be damaged or destroyed. At a place where the cells are destroyed, the surface resilience of the walls is decreased. Furthermore, as destruction of the cells progresses, the surface elastic force of the elastic roller is decreased. The stiffness on the Asker C scale is also decreased.
Changes to the fixing nip as the destruction of the cells in the elastic layer of the elastic roller progresses are explained with reference to FIG. 1A and FIG. 1B. FIG. 1A is an enlarged view of an area in the vicinity of the fixing nip before the destruction of the cells occurs. FIG. 1B is an enlarged view of an area in the vicinity of the fixing nip after the destruction of the cells takes place. As shown in FIG. 1A, an elastic roller 51 and a high-stiffness roller 52 push each other so that the shape of the elastic roller 51 is changed by large amount. Thereby, a fixing nip is formed. A surface moving direction of the elastic roller 51 and the high-stiffness roller 52 in the fixing nip is shown by an arrow A in FIGS. 1A and 1B. With the decrease in the surface elastic force of the elastic roller 51, the elastic layer portion of the elastic roller 51 forming a nip entrance B and a nip exit C is gradually separated from the high-stiffness roller 52. Therefore, it is assumed that the width W of the fixing nip is decreased, as shown in FIG. 1B.
When the width W of the fixing nip is decreased, it may not be able to adequately apply heat and pressure to a toner image on the recording medium passing the fixing nip. Therefore, there may be a problem in which the fixability of the toner image relative to the recording medium is deteriorated.